1. Technical Field of the Invention
The present invention relates to a vacuum pump capable of performing an ideal air removal action in pressure zones from low vacuum zones to high vacuum zones, and in particular relates to a multiple-type vacuum pump that possesses the function of a turbo-molecular pump which transfers air in high vacuum zones in a highly efficient manner, and the function of a screw type pump which transfers air in intermediary vacuum zones in a highly efficient manner.
Uses for this multiple-type vacuum pump invention include the emptying of the vacuum chamber of CVD equipment used in the manufacture of semiconductors.
2. Background Art
(The Screw Type Pump)
The screw type vacuum pump is one which is well-known among the conventional vacuum pumps. For example, a known one, described in the Japanese Laid-open Publication No. Sho 60-216089, is a kind of screw type pump used from low vacuum zones, known as sliding flow zones, to high vacuum zones, known as free molecule zones, and has superior air evacuation capabilities in low vacuum zones.
In other words, screw type pumps are highly efficient in the evacuation of air in low vacuum zones and are capable of high-speed air evacuation, but experience a decrease in air intake volume and a lowered air evacuation efficiency in high vacuum zones. However, in the vacuum pumps used as air evacuation units in the CVD equipment used in the manufacture of semiconductors, they are required to possess superior air evacuation characteristics not only in low vacuum zones, but also in high vacuum zones.
(The Technology of (J01) Described in Japanese Examined Patent Publication No. Hei-6-92799)
The following is a known prior art (J01) that is described in Japanese Examined Patent Publication No. Hei-6-92799 which aims to fulfill the above-mentioned requirement.
Described in this publication is a screw type vacuum pump whose air intake volume in high vacuum zones was increased for the purpose of improving its air evacuation efficiency in high vacuum zones.
The screw type vacuum pump mentioned in this publication has a groove width correlation {groove width/(thread width+groove width)} of 0.8-0.95 in its upstream edge portion in the direction of the air conveyance, and attempts to increase the air intake volume at its upstream edge by increasing the groove depth as it goes toward the upstream edge.
(Problems Relating to the Above-mentioned Prior Art (J01))
The air evacuation efficiency of the above-mentioned prior art did not actually increase in the high vacuum zones to the degree anticipated. The reasons for this are not clear, but probable causes such as the following can be assumed:
(1) When speaking of a screw type pump which has inferior air evacuation capacity in high vacuum zones, a reason for which its air intake volume in high vacuum zones does not increase can be explained by the design for vacuum pumps which originally came from screw type pump theories relating to direction of air transfer, from the upstream edge to the downstream edge. In other words, in turbo-molecular pumps, which have a high air transfer capacity in high vacuum zones, vanes are used for air transfer that, if strong, the thinner they are, the more volume of air they can take in, and the greater the air evacuation capacity. In screw type pump theory, however, no matter how the screw grooves and screw threads of the upstream edge portion were to be designed, the air intake capacity in high vacuum zones would not increase.
(The Turbo-Molecular Pump)
In contrast to the above-mentioned screw type pumps, turbo-molecular pumps, like those disclosed in patent publications and the like, such as in Japanese Examined Patent Publication No. Sho 50-27204, have superior air transfer characteristics in high vacuum zones.
That is, turbo-molecular pumps have a casing with a cylindrical inner surface, wherein lies a rotor which rotates around the rotary shaft of the shaft of the above-mentioned casing. On the inner surface of the above-mentioned casing, multiple fixed vanes (static vanes), arranged along the circumference, are arranged in a multi-level fashion at prescribed intervals in the direction of the shaft. On the outer surface of the above-mentioned rotor, multiple dynamic vanes, arranged along the circumference, are arranged in a multi-level fashion in the direction of the shaft. The above-mentioned static vanes and dynamic vanes are slanted in relation to the above-mentioned rotary shaft, and the tilt angle (vane angle) decreases from the upstream side to the downstream side.
Each level of each of the static vanes and dynamic vanes, which are placed in multi-level fashion at intervals in the direction of the above-mentioned shaft, is placed alternately in the direction of the shaft and organized in such a way as to take the air brought from the upstream edge going in the direction of air transfer and transfer it to the downstream side by virtue of the rotation of the above-mentioned dynamic vanes.
The air evacuation efficiency of a turbo-molecular pump such as this is high in high vacuum zones, but the problem with it is that its air evacuation efficiency in low vacuum zones is low.
Another problem is the use of large numbers of static vanes and dynamic vanes, which means a large number of parts, and a construction that is complex and costly. Still another problem is the ease with which the above-mentioned static vanes and dynamic vanes become dirty.
(The Multiple-Type Vacuum Pump)
Conventionally, multiple-type vacuum pumps that are a combination of the screw type pump and the turbo-molecular pump have been known, and it was hoped that a vacuum pump would be created capable of achieving a highly efficient air evacuation rate in low to high vacuum zones by bringing together the advantages of the above-mentioned screw type and turbo-molecular pumps. The technology for such multiple-type vacuum pumps, as in the following (J02), for example, are well known in the art.
(J02) is "An Easy to Understand Vacuum Technology" (Compiled and written by the Japan Vacuum Association, Kansai Branch; Published by the Japan Vacuum Association, Kansai Branch, pg. 91.about.99, published Jun. 23, 1995).
This (J02) prior art relates to a multiple-type vacuum pump that combines a screw type pump with a turbo-molecular pump, by placing the turbo-molecular pump on the upstream side of the screw type pump. The air taken in by the turbo-molecular pump on the upstream side is compressed and transferred to the screw type pump on the downstream side. For this reason, the screw type pump, which performs air evacuation with low efficiency in high vacuum zones, is able to take the air which has been compressed by the turbo-molecular pump on the upstream side and transfer it to the downstream side with great efficiency.
(Problems Associated with the Aforementioned Prior Art (J02))
The foregoing prior art (J02) requires that numerous static vanes and dynamic vanes be manufactured and placed at many levels in the direction of the shaft of the turbo-molecular pump and installed at prescribed locations. This results in high manufacturing costs.
Moreover, the structure of the turbo-molecular pump portion is complex, so that when it is used in CVD equipment as an air evacuation device, or when it expels a large quantity of reactive air which has not reacted with anything, it provides many places where side reaction product can easily stick and build up. Side reaction product sticks and builds up easily on the static vanes of turbo-molecular pumps, for example. The result is a multiple-type vacuum pump whose durability may be greatly deteriorated.
The applicants of the present invention learned from the problems associated with the above-mentioned conventional multiple-type vacuum pump, and have developed the following technology (J03) which has already been on the market for some time.
((J03) Multiple-type Vacuum Pump Shown in FIG. 17 and FIG. 18)
FIG. 17 is a drawing showing the side view of the rotor of the multiple-type vacuum pump which the applicants of this invention have developed and have had on the market for some time. FIG. 18 is a cross-sectional view taken along the line XVIII--XVIII of FIG. 17 above.
The multiple-type vacuum pump 01 in FIG. 17 and FIG. 18 has a casing with a cylindrical inner surface 02a and a rotor 03 which rotates around a rotary shaft of the shaft of the above-mentioned cylindrical inner surface. On the outer surface of the rotor 03 is formed an air transfer portion 04 which transfers air in the direction of the shaft at the time of rotation. In the above-mentioned air transfer portion 04, a screw type pump air transfer portion 05 is provided in a downstream portion in the direction of air transfer and a turbo-molecular type pump air transfer portion 06 of the upstream portion. Between the above-mentioned screw type pump air transfer portion 05 and the turbo-molecular type air transfer portion 06, there is provided a ring connector 07 formed as ring-shaped concave grooves.
The above-mentioned screw type pump air transfer portion 05 includes multiple screw threads 05a which are formed as a spiral and at circumferentially prescribed intervals in the above-mentioned downstream portion of the outer surface of the above-mentioned rotor 03, and screw grooves 05b formed in between each of the aforementioned multiple screw threads 05a. The above-mentioned turbo-molecular type pump air transfer portion 06 includes multiple vanes 06a formed at a slant in relation to the direction of the rotary shaft and at circumferentially prescribed intervals, and air transfer grooves 06b formed between each of the above-mentioned multiple vanes 06a.
For each of the above-mentioned vanes 06a: the thicknesses of the upstream edge vanes are made smaller than the width of the above-mentioned screw threads 05a, and the downstream edge is formed as a continuation at the upstream edge of the above-mentioned screw threads 05a, while the downstream edge of the base of the above-mentioned air transfer grooves 06b is formed so as to be continuous with the upstream edge of the base of above-mentioned screw grooves 05b.
In the (J03) multiple vacuum pump 01, constructed as described above, air which has been taken in from the upstream edge at the time of rotation is compressed by the turbo-molecular air transfer portion 06 and transferred to the upstream edge of the above-mentioned screw type pump air transfer portion 05. Different from the ordinary turbo-molecular pump, which has static vanes and dynamic vanes arranged in a multi-level fashion and placed alternately in the direction of the shaft, the turbo-molecular type pump air transfer portion 06 has only vanes that correspond to the first-stage dynamic vanes of the upstream edges of the ordinary turbo-molecular pump. For this reason, the turbo-molecular type air transfer portion 06 has a simple construction and is easy to manufacture.
(Problems to be Solved)
The multiple vacuum pump 01 of the prior art (J03) with its capacity as such has been sold on the market for nearly thirty years. The reason why the above-mentioned (J03) multiple vacuum pump has sold for over such a long period of time is because, over a period of many years, there has been no multiple vacuum pump with a capacity of epoch-making proportions, although various efforts have been made in the vacuum pump industry to develop a new multiple vacuum pump.
In the multiple vacuum pump 01 which uses the foregoing prior art (J03), the inventors of the present invention were interested in knowing what the air flow conditions, at the ring connector 07 part formed by the aforementioned ring-type concave groove, would be like if the turbo-molecular pump air transfer portion 06 on the upstream side were connected to the screw type air flow transfer portion 05, and conducted a simulation using a supercomputer in order to find out. The results of the simulation showed a smoother air flow and improved air transfer efficiency.
Based on the results of the simulation, a multiple-type vacuum pump was made such that the downstream edges of the vanes 06a and the air transfer grooves 06b of the turbo-molecular type pump air transfer portion 06 on the upstream side were connected to the upstream edges of the screw threads 05a and the screw grooves 05b of the screw type pump air transfer portion 05 on the downstream side. When this simulation was done, it was possible to achieve, in a verifiable manner, nearly double the capacity as compared to the above-mentioned conventional multiple-type vacuum pump (J03), (i.e., the capacity to discharge air in 1/2 the time of the conventional multiple-type vacuum pump) .